IWYU: usage of std::bitset in ui::AXEventGenerator requires bitset include.

Bug: 957519
Change-Id: I7b763c1d5dee77d3a2b12f5169d6ad86e5dcbc7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394164
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804853}

ui/accessibility/ax_event_generator.h

@@ -5,6 +5,7 @@
 #ifndef UI_ACCESSIBILITY_AX_EVENT_GENERATOR_H_
 #define UI_ACCESSIBILITY_AX_EVENT_GENERATOR_H_
 
+#include <bitset>
 #include <map>
 #include <ostream>
 #include <set>